Amperometric electrochemical cells have found widespread use for the detection of various gases in the environment, including use for the detection of carbon monoxide in the domestic environment.
As shown in FIG. 1, a typical cell 10 comprises two or three gas electrodes—a working or sensing electrode 12, a counter electrode 14 and, optionally, a reference electrode (not shown). In lower cost cells, the reference electrode is often omitted and the counter electrode serves as a combined counter/reference electrode. All three (two) of these electrodes comprise a very high surface area catalytic metal (or other conductive material) 12, 14 supported on a gas permeable membrane or substrate 18, 20. An electrolyte 16, for example an acid, is typically completely contained within a wick 17 at a condition of lowest humidity. The wick 17 acts to hold and supply electrolyte to the sensing electrode 12 such that the chemical reaction discussed below can occur. The cell 10 comprises a housing that defines a reservoir 11 for the electrolyte 16, in which is provided a diffusion hole 22 through which target gas can enter.
The basic principle of operation is that gas enters the cell 10 via the diffusion hole 22. The gas passes through the gas permeable membrane 18 of the sensing electrode 12 and contacts the catalyst 12. A reaction occurs at the interface of the catalyst 12 and the acid electrolyte 16 (i.e. at the intersection of gas, liquid and solid). This reaction releases or consumes a number of electrons (the precise number depending on the gas being sensed) that are supplied via an external circuit 24.
For example, in a cell configured to sense carbon monoxide (CO), the gas is oxidised at the surface of the sensing electrode 12 to produce positive hydrogen ions (H+) and negative electrons (e−):2CO+2H2O→2CO2+4H++4e−
The positive ions travel through the electrolyte 16 to the counter electrode 14, and the negatively charged electrons travel to the counter electrode 14 via the circuit 24. The reaction is completed at the counter electrode 14:4H++4e−+O2→2H2O
The overall reaction is:2CO+O2→2CO2 
The working electrode 12 is arranged such that gas from the environment enters the cell 10 and permeates through the substrate 18 where the ‘target gas’ present in the environmental gas (i.e. the gas that is to be sensed) reacts completely. Each gas molecule that reacts at the substrate 18 produces a fixed number of electrons (the number depending on the gas reacting) and the measurement of the current produced can then be related to the number of molecules of gas that has entered the cell and this is directly proportional to the concentration of the target gas in the environment. An ammeter, voltmeter or other circuit 24 can be used to measure/derive the current produced.
The counter and reference electrodes are, however, different. For measurement of a target gas in air, the counter/reference electrode 14 is generally configured to react with oxygen. This oxygen needs to contact the metal catalyst 14 at the interface of gas, liquid (electrolyte) and solid (catalyst). In theory, this oxygen could come from one of two places: either from air inside the cell 10 or from oxygen gas dissolved in the electrolyte 16. However, the solubility of oxygen in the electrolyte 16 is very low and the concentration of oxygen in air inside the cell 10 is relatively high and therefore oxygen from inside the cell 10 is consumed at the counter/reference electrode 14. This oxygen has to reach the metal/electrolyte interface 14, 20 by permeating through the permeable membrane 20 in order to reach the solid/liquid interface as previously described.
There are various factors that affect the efficient and reliable working and performance of an electrochemical cell.
Under certain conditions, or combinations of conditions, a degree of oxygen starvation can occur at the counter electrode 14 resulting in a decrease of the electrochemical efficiency due to the development of bias voltages. This results in a reduction of the current expected for a known concentration of the target gas, in turn resulting in an erroneous (low) reading of the gas concentration. These conditions include the orientation of the cell, the degree of hydration of the electrolyte 16 and high concentrations of the target gas for long exposure times. Furthermore, combinations of these conditions can increase the tendency for non-ideal performance, which is undesirable as it can lead to erroneous gas concentration measurements.
Acid electrolyte is generally hygroscopic in nature. That is, it will absorb or desorb water from the environment until the strength of the acid 16 is in equilibrium with the external atmospheric humidity. This absorption or desorption of water is accompanied with a change in the volume of the acid electrolyte 16. For the typical acid electrolyte used in these cells, sulphuric acid, the volume change from the typical lower operating humidity (15%) to the typical upper operating humidity (90%) can be as much as a factor of four. Therefore the design of the electrochemical cell has to be such that, at the highest operating humidity, the cell 10 is not so full that it leaks or bursts whilst, at the lowest humidity, the volume has to be large enough to ensure that the surfaces of both electrodes are fully wetted and that there is a continuous fluid path between the two electrodes (via the wick material).
Known cells all effectively comprise an axial reservoir, located either between or below the electrodes to accommodate the expansion in the electrolyte volume.
For reservoirs that are between the electrodes 12, 14, this large reservoir can produce a high internal resistance between the electrodes 12, 14 at lower humidities where the acid electrolyte is more dehydrated (and hence has a small volume) due to the relatively large distance between the electrodes and lower ionic conductivity.
For reservoirs that are below the lower electrode 14, there is sometimes the need for an additional thin piece of wick (or other wicking mechanism) to ensure that free acid is transferred into the ‘main’ wick 17. However, this wicking does not always occur effectively and can result in issues with repeated hydration/dehydration cycles. This is amplified by the fact that this thin piece of wick needs to be relatively long. Furthermore, positioning of this material during manufacture is complex and not easily amenable to automation.
In addition, the need for the axial reservoir imposes a certain physical structure on the cell and determines the height of the cell as approximately a minimum of 20 mm. For domestic carbon monoxide detectors, this height constricts the possible design options available for the detector.
Aspects and embodiments of the present invention have been designed with one or more of the foregoing in mind.